The present invention relates to a back projection type image display apparatus which obliquely projects an image light of a display device such as a CRT or LC to a screen from a back plane, and a screen therefor.
A back projection type image display apparatus which is of oblique projection type as shown in FIG. 1 has been known. An image light from a display device 1 such as a CRT is enlarged by a projection lens 2 and projected to a screen 5 from a back plane thereof obliquely at an incident angle .theta..sub.0, through reflection mirrors 3 and 4. Those elements are housed in a cabinet 6.
The oblique projection type can reduce the length ( of the cabinet 6 compared to a normal projection type in which the light is projected normally to the screen 5 at an incident angle .theta..sub.0 =0, but the image light appears .theta..sub.0 degrees below the horizontal angle and the incident angle expands in the periphery of the screen (particularly at the bottom in FIG. 1) and the image in that area is dark because of increased loss of light transmission. Thus, in order to emit the image light horizontally and deflect the image light gradually to reduce the light transmission loss, the use of the screen 5 which uses single or double eccentric center of prisms extending along a curve or arc deviates from a center of the screen) Fresnel lens sheet 5a or 5b as shown in FIGS. 2 and 3 has been proposed.
However, even in such screen 5, the following problems are encountered when the image light is reflected by mirrors 3 and 4 arranged as shown in FIG. 4.
If there is a plane or fresnel lens plane on a plane of the screen 5, a surface reflected light .beta. of the image light .alpha. from such plane may be directed to the mirror 4 and reflected thereby and directed back to the screen 5 at an incident angle .theta..sub.2 (which is smaller than the incident angle .theta..sub.1 of the image light .alpha.). Such internal reflection phenomenon creates a ghost image or flare, which in turn reduces the contrast of the image displayed on the screen.